The invention pertains to gas cleaning processes for removing contaminant gases and particulates from air, and more particularly to a process combining the operation of several distinct physical processes, together in one overall process, in which the distinct physical processes work together to optimize the cleaning from an effluent gas of contaminant particulates in the 0.01 micron to 0.1 micron diameter size range, as well as water-soluble contaminant gases. The field of application of the present invention process includes cleaning air of such particulates and water soluble toxic gases introduced into air by numerous industrial processes, particularly high temperature processes, as well as scrubbing of bacteriological agents from air, including viruses, bacteria, and spores including anthrax.
Although contaminants introduced into air by high temperature industrial processes are often initially of molecular size, upon cooling via mixing with ambient air or with cleaning water sprays some of such molecules rapidly condense to form clusters of molecules which in turn coagulate to form contaminant particulates in the size range from 0.01 micron to 0.2 micron diameter and larger.
Natural processes are generally ineffective in removing such particulates, as evidenced by the fact that the size distribution of the particulates found in xe2x80x9ccleanxe2x80x9d air has a maximum at about 0.02 microns diameter.
Although particulates in the size range from 0.01 micron to 0.2 micron usually represent only a small fraction of the total mass of particulates entering the atmosphere from industrial processes, such particulates can produce a significant opacity of the emitted plume, readily detected by emissions monitoring by environmental regulatory agencies, and can also affect the radiative heating and cooling of the atmosphere.
It has long been known in the gas cleaning arts, to clean an effluent gas of contaminants both in the form of particulates and water-soluble gases, by a variety of processes, including exposing the gas to copious quantities of water droplets which are subsequently collected by various means; and/or by using a means to charge the particulates if they are not already charged, and using charged droplets of charge polarity opposite to that of the particulates to collect the particulates; and/or by using electrostatic precipitating means to collect charged particulates and/or charged droplets; and/or by humidifying, cooling and supersaturating air containing particulates to allow condensation and growth of water droplets upon particulates serving as condensation nuclei. Such approaches are disclosed in numerous prior patents, including applicant""s prior patents and patents cited therein. See, e.g., patents cited in applicant""s own prior U.S. Pat. Nos. 6,156,098; 5,941,465; 5,147,423; and 4,345,916.
It is also known in the art, as disclosed in applicant""s U.S. Pat. No. 6,156,098, to use very highly charged water droplets in cleaning even uncharged particulates from an effluent gas, by invoking the monopole-dipole electrostatic attraction force which exists between a highly charged droplet""s monopole charge and the electric dipole induced by the droplet monopole in a nearby uncharged particulate, to draw the particulate and droplet together; and to use such charged droplets in collection of oppositely charged particulates.
Prior art approaches have had varying success for different particulate size ranges of particulates to be cleaned from an effluent gas. A difficult size range for achieving high collection efficiency has been the 0.01 micron to 0.1 micron diameter particulate size range.
There is a need, met by the present invention as detailed below, for a process invoking additional individual physical processes, in a combination not found to applicant""s knowledge in the prior gas cleaning art, to achieve highly efficient particulate gas cleaning in this relatively difficult 0.01 micron to 0.1 micron diameter size range.
And there is a need, also met by the present invention as detailed below, for specification of suitable operating parameters for the process, which will allow the process to achieve optimal results for that particulate diameter size range.
These needs are met in the present invention, by a two stage process in which the effluent gas is in the first stage brought to a high relative humidity and relatively high temperature, and then in a second stage exposed to copious quantities of cool water droplets, which have a substantial charge opposite to any particulate charge polarity, as detailed below, in such a manner as to invoke several distinct processes occurring in the second stage that can be made to cooperate, to enhance the particulate collection efficiency of the droplets.
These second stage processes, discussed in detail below, include first a thermo-phoretic effect, consisting of a thermo-phoretic force, exerted on particulates near the cool droplets, urging the particulates toward the droplets; a diffusio-phoretic effect, consisting of a diffusio-phoretic force, exerted on particulates near the droplets, also urging the particulates toward the droplets; and a condensation effect, whereby the cool droplets may cause the particulates to act as water condensation nuclei. Although these effects have not previously been invoked together in the gas cleaning arts to applicant""s knowledge, the physics of both the thermo-phoretic effect and the diffusio-phoretic effect were long ago explicated by Albert Einstein, in Physikalische Zeitschrift, Vol. 27, p. 1 et seq (1924).
As further detailed below, with a suitable combination of operating parameters, all three of these second stage processes may be made to act in concert to increase the particulate collection efficiency of the stage two cold water droplets.
The invention is an effluent gas cleaning process for removing varied kinds of particulates with high efficiency for particulates in the 0.01 micron to 0.1 micron diameter range, and for removing water-soluble gaseous contaminants, by first bringing the effluent gas to a relatively high temperature and humidity in a first stage, and then exposing the effluent gas to copious quantities of small cool charged water droplets in a second stage, which process is optimally carried out for particular combinations of the following operating parameters: the first stage relative humidity, being preferably greater than 95%; the temperature difference, between the first stage and the cool second-stage water droplets, being preferably at least 15 deg. F.; the stage two cool water droplet mass flow rate, being preferably at least equal to the effluent gas mass flow rate; the second stage cool water droplet diameter, being preferably being no larger than 200 microns; the second stage cool water droplets having an electrical charge, which is preferably is at least an appreciable portion of the theoretical maximum charge which may be carried by the droplets; the effluent having a travel time of preferably at least 2 seconds during exposure to the second stage cool droplets; and the particulates preferably having an electric charge, of opposite polarity to that of the second stage cool droplets. These combinations enhance effluent cleaning through the combined operation of up to four distinct physical processes which can occur during the second stage. Different embodiments of the invention involve varying combinations of said operating parameters, as recited in the claims.